Field of the Invention
The present invention concerns a cable, in particular for transport or distribution of electrical energy and insulating composition used therein.
In particular, the present invention concerns a cable, in particular for transport or distribution of electrical energy, preferably at medium or high voltage, comprising an insulating covering consisting of a polymeric composition comprising at least one polyolefin having improved electrical properties.
Within the scope of the present invention, xe2x80x9cmedium voltagexe2x80x9d means a voltage ranging from 5 to 35 kV, while xe2x80x9chigh tensionxe2x80x9d means a voltage greater than 35 kV.
At present, for the production of insulating layers of cables for transport of energy, cross-linked polyolefins are preferred. Typically, this polyolefin is a cross-linked polyethylene (XLPE).
Generally, the covering structure of such cables comprises three different layers of extruded material:
internal semiconducting layer;
insulating layer;
external semiconducting layer.
This covering structure is generally made by passing a metallic conductor through an extrusion head into which together flow three extruders (triple-head extrusion), which deposit the aforesaid layers onto the said metallic conductor in the order indicated above. In the case where it is desired to subject the said external layer to cross-linking, immediately after the extrusion the cable passes into a suitable device, also referred to as a vulcanising tube, where the said cross-linking is effected.
Generally the cross-linking is achieved by via radicals by thermal decomposition of organic peroxides, for example dicumyl peroxide, tert-butyl cumyl peroxide and the like, which are added to the polyolefin before the extrusion or injected directly into the extruder.
The extrusion temperature of the material which constitutes the insulating layer must not exceed the limit imposed by the decomposition temperature of the peroxide utilised. For example, when dicumyl peroxide is used, the temperature of the extruder is maintained below 130xc2x0 C. to avoid premature cross-linking of the insulating material.
Advantageously, the cross-linking process is performed at a temperature ranging from 200 to 400xc2x0 C. and the time necessary to achieve complete cross-linking of the insulating material varies from case to case depending on parameters well known to the technician of the field. Preferably, upon completion of the cross-linking, the cable is subjected to a degassing treatment, generally at a temperature of about 70-90xc2x0 C., to eliminate decomposition products of the peroxide such as, for example, methanol and water since their presence within the insulating layer can prejudice its performance in time. Then, the cable is cooled and collected on reels.
Finally, the cable is completed by the addition of a metallic screen, an external sheath and, in some cases, other protective coverings (armouring).
It is well known that, in general, dielectric strength (DS) values measured on the insulating layer of a real cable are markedly lower than the values obtained when the same insulating material is in the form of flat samples (plates). The reasons for these differences are not fully known but it is believed this limitation of the DS values on the cable may be due to the presence of defects (for example: voids, protrusions, metallic particles and contaminants), formed in the insulating layer during the extrusion process. The quantity of such defects would increase on increasing thickness of the insulating layer.
The presence of such defects would also be responsible for a considerable diminution in the lifetime of the cable.
In the art, various attempts have been described intended to limit the adverse consequences of such effects by adding small quantities of additives commonly referred to as xe2x80x9cvoltage stabilizersxe2x80x9d to the material which forms the insulating layer.
For example, EP-A-0 089 490 and EP-A-0 111 043 teach the use, as voltage stabilizer, of a mixture consisting of one or more divalent aliphatic alcohols having from 4 to 15 carbon atoms and of aliphatic or aliphatic/aromatic compounds, which are liquids or liquefy below 80xc2x0 C. and which contain alcoholic and/or ketonic functional groups. The said stabilizing mixture is added to the electrical insulating material in quantities of between 0.3 and 5% by weight. The insulating material is based on a polyolefin such as, for example, a low-density polyethylene cross-linked via peroxide. The aforesaid insulating material is said to show improved dielectric resistance over time even in the presence of moisture, offering protection against growth of the so-called xe2x80x9cwater treesxe2x80x9d and against occurrence of the so-called xe2x80x9celectrical treesxe2x80x9d. Both acetophenone and benzophenone are mentioned among the materials constituting the aforesaid stabilizing mixture.
DE 2 709 139 describes the use of a diaryl-ketonic carboxylic acid or of an ester thereof, in quantities that range from 0.1 to 5% by weight, as electrical voltage stabilizer in a polyolefin-based insulating material. The said voltage stabilizer is said not to interfere with the cross-linking process and would not be inactivated by peroxides only because it is generated xe2x80x9cin situxe2x80x9d by decomposition of the cross-linking agent itself. As an example of a voltage stabilizer, benzophenone-2-carboxylic acid, deriving from the decomposition of the cross-linking agent 3-t-butylperoxy-3-phenylphthalide, is in fact mentioned.
JP 47-28042 describes the use of benzophenone or benzophenone substituted with alkyl groups, aryl groups, halogen, or OH groups with poly alpha-olefins to improve dielectric breakdown strength in insulation of high-voltage cables or electrical machine. As an example of benzophenone derivatives 2-hydroxy-4-n-octyloxy benzophenone, 2,2xe2x80x2-dihydroxy-4-n-oxyloxybenzophenone, 2,2xe2x80x2-dihydroxy-4-n-dodecylbenzophenone, 2,2xe2x80x2-dihydroxy-4-n-dodecyloxybenzophenone, 2,2xe2x80x2-dichloro-4-methylbenzophenone, 2,2xe2x80x2-dihydroxy-4-n-butyloxybenzophenone, 2-bromo-4-methylbenzophenone are mentioned.
GB 1304112 describes a method for the polymerization of monomers and crosslinking of polymers with radiation. Insulation on electrical conductors can be crosslinked with the predominantly continuum visible light radiation from the radiation source by exposing the insulated conductor to the predominantly continuum visible light radiation. The rate and extent of crosslinking can be enhanced by blending the crosslinkable polymer with photosensitizers among which benzophenone, 3- or 4-methylbenzophenone or 3- or 4-methoxybenzophenone are mentioned.
Therefore, the need to produce electrical cables equipped with a polyolefin insulating covering having improved electrical properties, in particular high values of dielectric strength and stability over time, but using conventional cross-linking systems, is still keenly felt.
The Applicant proposed to satisfy this need by adding a voltage stabilizer having the following group of properties to the material that constitutes the insulating layer of the cable:
ability to increase the lifetime and the dielectric strength of the insulating layer without substantially altering the other electrical properties required for an insulating material, in particular low dielectric loss (tandelta) values;
high stability in the insulating material over time thanks to a reduced ability to migrate towards the external surface of the insulating layer itself. In fact, the migration of the additive involves a loss of the stabilizing effects over time, above all in the interface zone between the internal semiconducting layer and the insulating layer where probability of partial discharges is the greatest;
substantial inertness towards commonly used cross-linking agents, in particular organic peroxides, thus avoiding phenomena of inhibition of the cross-linking reaction and/or alteration or destruction of the additive itself during the cross-linking process;
chemical and physical properties which make the use of the additive in the cable production process convenient and safe, in particular suitable boiling point (B.Pt.), melting point (M.Pt.) and ignition temperature (flash point) values, and substantial lack of toxicity.
The Applicant has found that this objective is achieved by using, as voltage stabilizers, benzophenones substituted with functional groups as defined hereinbelow.
Thus, according to a first aspect, the present invention concerns an electrical cable comprising at least one conducting element, at least one polyolefin-based insulating covering layer, in which the said covering layer comprises at least one voltage stabilizer, characterized in that the said voltage stabilizer is a benzophenone substituted with at least one group selected from alkyl, arylalkyl, and alkylaryl, and in that the said group:
a) is linked to a phenyl ring of the benzophenone directly or via an oxygen bridge (xe2x80x94Oxe2x80x94);
b) contains, optionally, one or more oxygen bridges (xe2x80x94Oxe2x80x94); and
c) is optionally linked to a phenyl ring of at least one other benzophenone group,
provided that when said at least one group is an alkyl, optionally substituted, the carbon atom of the said alkyl which is directly linked to a phenyl ring of the said benzophenone is tertiary.
In a preferred embodiment the total number of carbon atoms of said at least one group is greater than 8 The presence of at least one group with more than 8 carbon atoms improves compatibility of the voltage stabilizer with the polyolefin-based insulating material, thus decreasing exudation (migration) of the additive from the insulating material.
According to a preferred aspect, the voltage stabilizer is a substituted benzophenone of formula (I): 
wherein:
R1, R2, R3 and R4, equal or different from each other, are selected from:
hydrogen;
C6-C14 aryl, substituted with at least one group selected from C1-C30 alkyl, C1-C30 alkoxy and C6-C14 aryloxy;
C1-C30 alkyl, optionally substituted with at least one group selected from C6-C14 aryl, C1-C30 alkoxy and C5-C14 aryloxy;
C1-C30 alkoxy, optionally substituted with at least one group selected from C6-C14 aryl and C6-C14 aryloxy;
C6-C14 aryloxy, optionally substituted with at least one group selected from C6-C14 aryl, C1-C30 alkyl, C1-C30 alkoxy;
a group of formula: 
xe2x80x83wherein:
R1xe2x80x2, R2xe2x80x2, R3xe2x80x2 and R4xe2x80x2, equal or different from each other, are selected from the same groups indicated above for R1, R2, R3 and R4; and
xe2x80x94Qxe2x80x94 is a group of formula xe2x80x94Oxe2x80x94R5xe2x80x94Oxe2x80x94, where R5 is selected from:
C1-C30 alkylene, optionally substituted with at least one group selected from C6-C14 aryl, C1-C30 alkoxy and C6-C14 aryloxy;
C6-C14 arylene, optionally substituted with at least one group selected from C1-C30 alkyl, C1-C30 alkoxy and C5-C14 aryloxy;
wherein, optionally, the alkyl and alkoxy groups have one or more oxygen bridges (xe2x80x94Oxe2x80x94) along the aliphatic chain;
xe2x80x83provided that:
at least one of the substituents R1, R2, R3 and R4 is different from hydrogen; and
when said at least one group is an alkyl, optionally substituted, the carbon atom of the said alkyl which is directly linked to a phenyl ring of the said benzophenone is tertiary.
According to the present invention, when the benzophenone of formula (I) according to the present invention is used in a cross-linking system, R1, R2, R3, R4, R1xe2x80x2, R2xe2x80x2, R3xe2x80x2and R4xe2x80x2 have the meanings indicated above, provided that when at least one of these is a C1-C30 alkyl, optionally substituted as indicated above, the carbon atom of the said alkyl which is directly linked to a phenyl ring of the said benzophenone is tertiary.
It has in fact been found that these compounds are more stable towards the cross-linking agents, in particular towards the peroxides, than corresponding compounds having alkyl substituents which have benzylic hydrogens. In this way, a possible interaction between the voltage stabilizer and the cross-linking agent is substantially avoided.
Preferably, the said polyolefin is a polyolefin cross-linked via radicals, still more preferably cross-linked via peroxides.
Typically, the said voltage stabilizer has a boiling point higher than the extrusion temperature of the insulating material. Further, to guarantee an optimal dispersion of the stabilizer in the polymeric material, the said stabilizer preferably has a melting point lower than the extrusion temperature of the insulating material.
Typically, using polyethylene as the insulating material, the said voltage stabilizer preferably has a boiling point higher than 180xc2x0 C. and a melting point lower than 150xc2x0 C. In order to avoid handling problems with the said voltage stabilizer, particularly in the cable production stage, it preferably has an ignition temperature (flash point) higher than 110xc2x0 C.
Preferably, the quantity of the said voltage stabilizer in the insulating layer ranges from 0.1 to 5% by weight relative to the total weight of the said insulating layer, and still more preferably it ranges from 0.5 to 2%.
In a preferred embodiment, the said insulating covering layer of the cable according to the present invention is cross-linked using a peroxide selected from the group comprising dicumyl peroxide, tert-butyl peroxide, tert-butyl cumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, xcex1,xcex1xe2x80x2-bis(tert-butylperoxy)diisopropylbenzene, and the like, or mixtures thereof.
Besides the said polyolefin, the said peroxide and the said voltage stabilizer, the insulating layer of the cable according to the present invention can also comprise other conventional additives such as, for example, antioxidants, processing aids, lubricants, pigments, xe2x80x9cwater-tree retardantxe2x80x9d additives, and the like.
According to a second aspect, the present invention concerns a polyolefin-based insulating composition comprising at least one voltage stabilizer, characterized in that the said voltage stabilizer is a benzophenone substituted with at least one group selected from alkyl, arylalkyl, and alkylaryl, and in that the said group:
a) is linked to a phenyl ring of the benzophenone directly or via an oxygen bridge (xe2x80x94Oxe2x80x94);
b) contains, optionally, one or more oxygen bridges (xe2x80x94Oxe2x80x94); and
c) is optionally linked to a phenyl ring of at least one other benzophenone group,
provided that when said at least one group is an alkyl, optionally substituted, the carbon atom of the said alkyl which is directly linked to a phenyl ring of the said benzophenone is tertiary,
and with the proviso that said substituted benzophenone is different from 3- or 4-methoxy-benzophenone. Preferably the said voltage stabilizer is a substituted benzophenone of formula (I) as defined above. In a preferred embodiment the total number of carbon atoms of said at least one group is greater than 8.
Examples of C6-C14 aryl groups are phenyl, naphthyl, anthracyl, biphenyl and the like. Preferably, the aryl group is phenyl.
xe2x80x9cC1-C30 alkylxe2x80x9d means an aliphatic group, linear or branched, of formula xe2x80x94CnH2n+1, where n is an integer from 1 to 30, or a cycloalkyl group of formula xe2x80x94CmH2m, where m is an integer from 3 to 30.Examples of C1-C30 alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, 1-methyl-butyl, 1-ethyl-butyl, 1,1-dimethyl-butyl, 1-methyl-1-ethyl-butyl, 1,2-dimethyl-butyl, 1-methyl-2-ethyl-butyl, 1,3-dimethyl-butyl, cyclobutyl, cyclohexyl, 2-methyl-cyclohexyl, 3-methylcyclohexyl, 4-methylcyclohexyl, and the like, and superior homologues thereof.
Preferably, the C1-C30 alkyl group is a tertiary alkyl group such as, for example, tert-butyl, 1,1-dimethyl-butyl and 1-methyl-1-ethyl-butyl.
With xe2x80x9cC1-C30 alkoxyxe2x80x9d it is meant
a group of formula xe2x80x94Oxe2x80x94CpH2p+1 where p is an integer ranging from 1 to 30, or
a chain of general formula
xe2x80x94Oxe2x80x94(CH2O)q(CH2CH2O)r(CH(CH3)CH2O)s(CH2CH(CH3)O)txe2x80x94R6 
in which R6 is a C1-C4 alkyl and q, r, s and t are 0 or an integer ranging from 1 to 30, provided that the total number of carbon atoms ranges from 2 to 30.
Examples of alkoxy groups are xe2x80x94OCH3, xe2x80x94OC2H5, xe2x80x94Oxe2x80x94(CH2)tCH3 (t integer between 2 and 24), xe2x80x94Oxe2x80x94CH(CH3)CH3, xe2x80x94Oxe2x80x94C(CH3)3, xe2x80x94OCH2OCH3, xe2x80x94OCH2OCH2OCH3, xe2x80x94O(CH2O)4CH3,xe2x80x94OCH2CH2C2H5, xe2x80x94Oxe2x80x94CH2OCH2OC2H5, and the like, and superior homologues thereof.
Preferably, alkoxy is a group of formula xe2x80x94Oxe2x80x94CpH2p+1 where p is an integer ranging from 1 to 20, even more preferably from 5 to 20.
Examples of C6-C14 aryloxy groups are phenyloxy, naphthyloxy, p-phenyl-phenyloxy and the like. Preferably, aryloxy is a phenyloxy group.
Preferably, xe2x80x94Qxe2x80x94 is a group of formula xe2x80x94Oxe2x80x94(CH2)uxe2x80x94Oxe2x80x94, where u is an integer ranging from 1 to 24, even more preferably from 5 to 15.
Typical examples of substituted benzophenones according to the present invention are those selected from the group comprising:, 2,4-di-octyloxy-benzophenone, 4(1,1-dimethyl-1-tridecyl) benzophenone, 4,4xe2x80x2-di-tertbutyl-benzophenone, 4-dodecyloxy-benzophenone, 1,12-di-4-benzoylphenoxy dodecane, 4-octadecyloxy-benzophenone and 4,4xe2x80x2-di-dodecyloxy-benzophenone.
Preferably, the substituted benzophenone of the present invention is selected from 2,4-dioctyloxy-benzophenone, 4(1,1-dimethyl-1-tridecyl)benzophenone, 4,4xe2x80x2-di-tert-butylbenzophenone, 4-dodecyloxy-benzophenone, 1,12-di-4-benzoylphenoxy dodecane, 4-octadecyloxy-benzophenone and 4,4xe2x80x2-di-dodecyloxy-benzophenone.